1. Field of the Invention
The invention relates to a technology for fabricating a coil unit for a linear motor or a single coil thereof through line material (conductive wire) winding.
2. Description of the Prior Art
Linear motors are simple in structure, low in parts count, and capable of driving their moving bodies linearly even with precision and speed. Accordingly, the linear motors find wide use as linear drive units or positioning devices in any fields such as exposure devices for semiconductor manufacturing and high precision machine tools.
In general, a linear motor is composed of a magnetic pole unit having magnets and a coil unit having coils. Either one of the units is fixed to a base as a fixed body, and the other is coupled to a moving table or the like as a moving body. The magnetic pole unit and the coil unit are opposed to each other with a constant gap therebetween. When magnetic force is created between the two units, this magnetic force functions as thrust to drive the moving body without contact while maintaining the above-mentioned gap.
For one form of the linear motor, a direct-current linear motor of multi-pole/multi-phase type has been disclosed. In this linear motor, a magnet unit is composed of a plurality of N/S poles that are arranged so that adjoining poles have opposite polarities. Moreover, a plurality of single coils are connected to form a single coil unit as a whole.
Each of the single coils constituting the coil unit has the overall shape of a nearly rectangular ring. Among the four sides of this rectangular, the two sides opposed to each other across the traveling direction function as effective conductors which contribute to the thrust production in a moving body of a linear motor. The other two sides make connecting conductors for connecting the effective conductors. The connecting conductors do not particularly contribute to the thrust production in the linear motor.
Suppose that the magnetic flux density acting on the effective conductors is B (T), the current flowing through the effective conductors is I (A), and the length of the effective conductors is L (m). The thrust F (N) of the linear motor is given by F=BIL. Then, assuming that the number of turns of each single coil is n, F is represented as F=BniL. Where i is the per-wire current.
It can be seen from above that at given dimensions or specifications of the component members, the maximization of the thrust F requires that each single coil be increased in the number of turns.
Generally, a wire can be wound a plurality of times to form a coil by using the method of: preparing a so-called “winding former” consisting of a male piece and a female piece in conformity with the shape of the coil; coupling these pieces to form a space for the wire to be wound on; and winding the wire around the winding former (over and over) sequentially.
For the case of a coil unit for a linear motor, however, the single coils are arranged closely in a traveling direction. This generally requires that each single coil have its connecting conductors bent sharply from the effective conductors. Therefore, the simple method of winding as described above has the problem that the “bents” are extremely hard to form by means of the winding former's configuration alone.
Now, brief description will be given of a technology according to Japanese Patent Laid Open No. 2001-67955. The description is given by way of example for the sake of a proper understanding of the foregoing problem to be solved by the present invention or the validity of the present invention.
This technology uses a single coil 2 of saddle shape as shown in (b) of FIG. 10, formed by sharply bending connecting conductors 6 at approximately 90 degrees with respect to effective conductors 4. As shown in (c) of FIG. 10, single coils 2 (2a, 2b) of such saddle shape are closely arranged in order with little gap therebetween. Here, the single coils 2a having their connecting conductors 6 bent to the right with respect to the traveling direction A and the single coils 2b having their connecting conductors bent to the left get into between the effective conductors 4a, 4b of the other parties each other. The single coils 2 are interconnected according to the specifications of a coil unit 10, thereby forming a single coil unit for one linear motor.
When the single coils 2 are driven with a three-phase current, currents having 120-degree differences in phase are passed through adjoining single coils 2 to make a U-V-W three-phase coil unit. Each single pole, a constituting unit of a linear motor, is defined as a part from one N/S pole of the magnet array to a next N/S pole. The number of the single coils 2 corresponding thereto is three; or the U, V, and W phases (per pole).
Conventionally available coil units for a linear motor are formed by combining two types of single coils, more specifically, ones having their connecting conductors bent to the right or left with respect to a traveling direction and ones having no bent. It is characteristic of the coil units to be seen the three phases of coils in a cross section perpendicular to the pole pitch direction (coincident with the direction A). In contrast, this coil unit 10 includes a single type of single coils 2 alone, which are simply distributed to either side and combined with each other to form the coil unit 10. This means a major characteristic that only two phases of single coils 2 appear in that cross section. These single coils 2 or the coil unit 10 successively offers a number of highly beneficial advantages for reasons including the following. That is, the coil unit 10 is formed with the single coil 2 of one type alone; the length Wo of the connecting conductors 6 is made as short as possible with respect to the length Lo of the effective conductors 4; and the effective conductors 4 are arranged with no gap formed therebetween.
Nevertheless, each single coil 2 in this technology is configured so that a pair of connecting conductor 6 bend at approximately 90 degrees “in the same direction” with respect to the effective conductors 4. The single coils 2 of such configuration are extremely hard to fabricate by “the method of winding by using a conventional winding former,” in fact.
Even if managed to wind, it is extremely difficult to secure the wire at a proper winding angle to the winding former in forming each of the pair of connecting conductors. If the winding tension is increased to prevent the production of slack and the like, a desired coil shape cannot be obtained due to accumulated twists. Besides, the wire density (space factor) varies from place to place, resulting in poor magnetic performance. In particular, when the number of turns n of each single coil is increased for the sake of greater thrust, each side of the rectangular becomes greater in cross-sectional area. This eventually precludes the winding itself.
On that account, the Japanese Patent Laid Open No. 2001-67955 has also proposed a technology of: “initially winding a rectangular wire of t in thickness a plurality of times within the same plane to form a rectangular coil sheet 3; bending a pair of connecting conductors 6 thereof at approximately 90 degrees in the same direction with respect to the effective conductors 4 to form a coil sheet 3a in a U-shape; and preparing a plurality of such U-shaped coil sheets 3a having slight differences in width and bent positions, and laminating the same into one single coil 2” as shown in (a) of FIG. 10.
Nevertheless, there is no denying that the fabrication of a single coil by laminating a plurality of coil sheets having slight differences in width and bent positions is disadvantageous in terms of cost and flexibility for changing design.